Flexible polyurethane foams (hereinafter, occasionally referred to as “flexible foam”) are generally produced through a reaction of a polyol and a polyisocyanate in the presence of a catalyst, a blowing agent, and, as necessary, a foam stabilizer, a flame retardant, or a crosslinking agent, etc.
For example, flexible foams for seat cushions for automobiles are required to have high durability to reduce the change in the viewpoint of a driver due to the reduction of the thickness even in a long driving, from the viewpoint of safety. For recent seat cushions, on the other hand, it is required to lower the modulus of repulsion elasticity of a foam for the purpose of reducing the vibration transmitted from the surface of a road and to lower the foam density to the maximum extent from the viewpoint of cost reduction. However, such reduction of repulsion or lowering of density is known to significantly deteriorate the foam durability, and a technique to achieve durability, a ride quality, and economic efficiency in combination has been required.
For a means to solve the problems, for example, a flexible foam using a mixture of tolylene diisocyanate (hereinafter, occasionally referred to as “TDI”) and polyphenylene-polymethylene polyisocyanate (hereinafter, occasionally referred to as “Poly-MDI”) has been proposed (e.g., see Patent Literature 1).
However, flexible foams containing TDI as a main raw material (hereinafter, occasionally referred to as “TDI-based flexible foam”) are known to have a higher modulus of repulsion elasticity than flexible foams containing diphenylmethane diisocyanate (hereinafter, occasionally referred to as “MDI”) as a main raw material (hereinafter, occasionally referred to as “MDI-based flexible foam”), and a large amount of the modulus of repulsion elasticity needs to be lowered to achieve a sufficient vibration absorbability.
Examples of means to lower the modulus of repulsion elasticity include lowering of the crosslinking density or elevation of the glass transition temperature for a resin, and reduction of the air permeability of a foam; however, these are all known to deteriorate the durability of a flexible foam, and as a consequence TDI-based flexible foams cannot achieve a satisfactory vibration absorbability owing to a low modulus of repulsion elasticity and a high durability owing to a low hysteresis loss rate in combination.
Further, TDI-based flexible foams have a large density difference between the skin layer and the core layer in the foam, which results in a harder surface texture and a poorer ride performance than a foam having corresponding 25% compression hardness and hysteresis loss rate. Furthermore, use of TDI, which has a high vapor pressure, is known to deteriorate the working environment of a place for flexible foam production.
In contrast, MDI-based flexible foams are believed to be able to achieve vibration absorbability and durability in combination in a high density region, in which the modulus of repulsion elasticity is generally low, more easily than TDI-based flexible foams.
However, MDI has a lower isocyanate group content (NCO content) per unit weight than TDI, and it is required to blend a large quantity of water to lower the density of a flexible foam. In a low density region of lower than 55 kg/m3, which is required for recent seat cushions, the number of rigid urea linkages generated through a reaction of an isocyanate and water increases to decrease the restoration of deformation of a resin, and as a result the durability is deteriorated and a satisfactory feeling, a feature of MDI-based foams, is also lost. Accordingly, an MDI-based flexible foam having a hysteresis loss rate, which is an indicator of durability, of lower than 28%, has not been provided in the above-mentioned low density region.
As a method for producing a flexible foam having a high resilience (high repulsion) at a density of 40 to 45 kg/m3, for example, a method has been proposed in which an unmodified MDI containing 81 to 100% of diphenylmethane diisocyanate containing 40 to 60% of 2,2′-diphenylmethane diisocyanate and 2,4′-diphenylmethane diisocyanate therein, and a polyol composition containing a polyether polyol having a nominal average hydroxyl functionality of 2 to 6 and an average equivalent weight of 200 to 600 and containing at least 60% by weight of an oxyethylene group are reacted together (e.g., see Patent Literature 2).
In this method, however, another active hydrogen group-containing compound including water and the polyol having an oxyethylene unit react with the isocyanate in competition, and thus the reaction of the terminal hydroxy group of the polyol having an oxyethylene unit is not completed presumably, and an effect of lowering the hysteresis loss rate cannot be achieved sufficiently.